Running on empty: the pros and cons of fasting

Fasting restricts calories and may benefit your body. Is it a safe way to lose weight?

Something about the way Americans eat isn't working -- and hasn't been for a long time.

The number of obese Americans is now greater than the number who are merely overweight, according to government figures released last month. It's as if once we taste food, we can't stop until we've gorged ourselves.

Taking that inclination into account, some people are adopting an unusual solution to overeating. Rather than battling temptation in grocery stores, restaurants and their own kitchens, they simply don't eat. At least not at certain times of the day or specific days of the week.

Called intermittent fasting, this rather stark approach to weight control appears to be supported by science, not to mention various religious and cultural practices around the globe. The practice is a way to become more circumspect about food, its adherents say. But it also seems to yield the benefits of calorie restriction, which may ultimately reduce the risk of some diseases and even extend life. Some fasters, in fact, ultimately switch from regular, if comparatively rare, periods of hunger to permanent deprivation. They limit calories all the time.

"There is something kind of magical about starvation," says Dr. Marc Hellerstein, a professor of endocrinology, metabolism and nutrition at UC Berkeley, who studies fasting.

Adds Mark P. Mattson, chief of the laboratory of neurosciences at the National Institute on Aging: "In normal health subjects, moderate fasting -- maybe one day a week or cutting back on calories a couple of days a week -- will have health benefits for most anybody." Mattson is among the leading researchers on the effects of calorie restriction and the brain.

Not all nutrition professionals see the merits of fasting. Some think of it as a recipe for disaster, setting up a person for binge eating and metabolic confusion.

Ruth Frechman, a registered dietitian in Burbank and spokeswoman for the American Dietetic Assn., says she frequently sees such extreme strategies backfire. "You're hungry, fatigued, irritable. Fasting is not very comfortable. People try to cut back one day and the next day they're starving and they overeat."

And from a biological standpoint, they say, fasting can be helpful whether someone is overweight or normal weight.

"We're brilliant at this," Hellerstein says, referring to humans' physical reaction to not eating. "We're not good at responding to too many calories, but we're very good at responding to fasting. Fasting, in itself, is not an unhealthy process."

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Benefits to body

During fasting, almost every system in the body is "turned down," Hellerstein says. The body changes how it uses fuel. Certain hormone levels fall. Growth stops. Reproduction becomes impossible.

"By the end of three weeks of fasting you are a completely different metabolic creature," he says.

"It affects many, many processes -- but in a somewhat predictable way that takes you toward disease prevention."

Put simply, intermittent fasting appears to offer the same advantages as long-term calorie restriction -- defined as eating at regular times but consuming 25% to 30% fewer calories than what is recommended for that person based on age, size and gender (see accompanying article). People who eat this way tend to do so by filling up on nutrient-dense but low-calorie foods. They get all the protein, fiber, vitamins and minerals the body absolutely needs -- and very little else.

With intermittent fasting, "the idea is that maybe you can trick the system to think it's starving, but not make it starve every day," Hellerstein says.

Researchers aren't sure why the body apparently benefits from a state of mini-starvation. One theory is that the process produces just enough stress in cells to be good. "What our evidence suggests is that nerve cells in animals that are on dietary energy restriction are under mild stress," Mattson says. "It's a mild stress that stimulates the production of proteins that protect the neurons against more severe stress."

What they do know is that occasionally going without food or reducing calories daily makes the body more sensitive to insulin, which helps maintain normal blood sugar levels. And animal studies suggest calorie restriction may reduce the risk of cancer by slowing the growth of abnormal cells.

"We've been finding that putting an animal on a reduced-calorie diet for a couple of weeks dramatically slows cell proliferation rates," Hellerstein says. "This is the case in pretty much every tissue you look at: prostate, skin, colon, liver, lymphocytes."

Intermittent fasting and calorie restriction have also been shown in animals to reduce cognitive decline in diseases such as Alzheimer's disease and Parkinson's disease, Mattson says.